The invention concerns a method and a device for safe disconnection of electric drives configured as frequency-controlled three-phase crane motors, in which signals from at least one sensor, which detects the motions produced by the electric drive, are compared in an evaluation unit with a preselected limit value and a safe disconnection is achieved via the evaluation unit to produce the function of an end switch element in the event that the preselected limit value of the electric drive is exceeded.
In lifting mechanisms of hoisting equipment, a highly reliable insurance against dropping of the load in the event of mechanical flaws in the lifting mechanism is required. For this reason, the legislature as well has mandated extensive safety measures, such as those set forth, for example, in the European license EN954.T1 for lifting mechanisms. In order to achieve the high safety standard of EN954.T1, Category 3, cranes at present employ path limiters, such as transmission limit switches, incremental pickups for speed governing of the drive, as well as centrifugal switches to detect overspeeding of the drum of the lifting mechanism. At a given overspeeding, the centrifugal switch shuts off the lifting mechanism and activates its brake.
The drawback to this layout is that the lifting mechanism can only be shut off at a relatively high maximum number of revolutions, because different velocities, which are even higher than this, can occur during operation with a load. But when the lifting mechanism is working with various velocities, depending on the load, the centrifugal switch recognizes the overspeeding only when the maximum setpoint velocity is attained. As a result, it can happen that the disconnection by overspeeding is delayed for a needlessly long time when operating at maximum load and moving in dependence on the load, that is, when traveling with low velocity and heavy weight, with the danger that a heavy load with unwanted acceleration can only be halted with great difficulty at times.
Other end switches traditionally used for the safe disconnection of a crane's lifting mechanism are so-called transmission limit switches, which detect the number of revolutions of the drum of the lifting mechanism and provide for an end shut-off when only the required minimum number of turns is left on the cable drum as the cable is being paid out.
Furthermore, in the state of the art at present, electromechanical end switches are arranged on the axle of a crane to limit the path of crane trolleys or gantry cranes, for example. These end switches often have several trip cams and they are activated by a mechanical driver at a particular position in the path of the crane trolley or the gantry crane as this position is neared.
However, this technique has its disadvantages. If several end switches are present on a crane axle, for example, if several speed levels have to be monitored or switched off for said crane axle, this can result in malfunctioning if the driver fails or gets misadjusted. Likewise, the switch can become “overridden” when the axle is traveling at high velocity, such as is possible in the case of cross roller switches. The stationary mounted cross roller switch is activated by a movable driver. The switch then executes a rotary motion. If, now, the driver is moved quickly across the cross roller switch, the latter may become stripped or be overridden. The expected clear switching signal is then not recognized, and possibly several signal conditions one after the other will be reported. Additional drawbacks are the large adjustment time needed for this end switch with its drivers in terms of position and the switching hysteresis.
The high cost of materials and wiring are additional drawbacks when one uses a contactor-based logical control and storage-programmable control systems, since for safety considerations the contacts of the end switch very often need to be hooked up in parallel, in order to ensure a “hard-wired” disconnection.
From German patent DE 44 40 420 C2, a mechanism is known for monitoring and/or controlling the number of revolutions of an electric drive. The electric drive consists of an induction motor, hooked up to an alternating or three-phase current network and outfitted with a brake device. A revolution counter is connected to the induction motor and its pulses are taken to a speed control mechanism. The speed control mechanism acts on the induction motor via a frequency converter and it controls or monitors the induction motor such that the maximum number of revolutions is limited for different loads, so that a dangerous movement of the load due to exceeding of the available braking moment is prevented and loads once lifted are held securely. Normally, for safety reasons, an electromechanical centrifugal switch is also present, being triggered at a preset maximum number of revolutions.
Furthermore, from German application DE 196 12 423 A1, there is known a safety and control system for crane equipment with at least one control system and corresponding safety circuits. By definition, the control systems are, in particular, interlocks, which serve for comfort and also possibly support the safety circuits. By safety circuits are meant those interlocks which serve the parking safety of a crane and the protection of the persons. When necessary, the safety circuits bring about a coerced disconnection of the power supply to the particular drive of the crane equipment. If the coercive signal of an end switch is processed in an electronic control system, an error will be accordingly detected there and then be reported or used to disconnect the drive. In order to satisfy existing safety standards, it is proposed to provide two redundant storage-programmable control systems in addition to the actual control system for the safety circuits, being connected via a redundant bus system to likewise redundant sensors. The sensors serve to detect path, load, or speed signals. This safety and control system thus works with a redundant electronic detection and a redundant electronic evaluation of the signals from the sensors. Any resulting disconnection of the drive or activation of a brake will not be redundant, but rather occur via one of the two storage-programmable control systems.